In an electrophotographic method, a toner in a developer is stuck to electrostatic latent images formed on a photoreceptor to form a toner image and after transferring the toner image onto a transfer material, the toner image is fixed by heating to form an image.
On the other hand, in the printer and copying machine using an electrophotographic method, color printing has been progressed and from the improvement of the resolving power of the apparatus, the electrostatic latent images have become minute. Particularly, in a full-color copying machine of performing the development-transfer-fixing of digital latent images with color toners, a high image quality to some extent has been attained by employing color toners having small particle sizes of from 7 to 8 .mu.m.
In these printers and copying machines, as a method of fixing unfixed toner images attached to the surface of a transfer material, there are a contact fixing system such as a pressure fixing system using only a pressure roll at normal temperature, a hot roll system using a heat roll (hereinafter, is referred to as "a contact heating-type fixings system"), etc., and a non-contact fixing system such as an oven fixing system, a flash fixing system, an electromagnetic wave fixing system, a solvent fixing system, etc.
In the contact fixing systems, the contact heating-type fixing system has been widely used at present because the system has a good heat efficiency, is relatively compact, is liable to control temperature, and has a high reliability.
The contact heating-type fixing system has been the mainstream in full-color copying machines and in this case, the material constructions of the heating roll and the press roll used for heat fixing, the fixing nip structure, etc., determine the characteristics of images, such as the coloring property and the image gloss of full-color images, the expansion of images, etc., and the releasing property of transfer materials from rolls at fixing.
For fixing images, it becomes necessary to sufficiently melt toners on a transfer material. To ensure the releasing property of a transfer material from a heating roll which is brought into contact with molten toners on the transfer material, it is frequently performed to supply a large amount of a silicone oil onto the surface of the heating roll. However, in this case, there occurs a problem that after fixing, writing onto the transfer material with a pencil, a ball pen, etc., becomes difficult. To solve such a problem, a so-called oilless fixing roll system of reducing the supplying amount of the silicone oil or not using the silicone oil has been investigated. However, because in these systems, the releasing property is gradually lowered with the increase of the number of prints, an increased improvement of the durability until the generation of an offset phenomenon has been desired.
Now, in a full-color copying machine, in the toners of cyan, magenta, and yellow, by overlapping a single color, two colors, or three colors, a full-color image is formed. Accordingly, because the toner amount of even each single color in the image area ratio 100% of a toner image formed on a toner transfer material (hereinafter, referred to as "transferred image") is usually from about 0.6 to 0.9 mg/cm.sup.2, in the image of a process black formed by overlapping toner images of three colors, the toner amount becomes from 1.8 to 2.7 mg/cm.sup.2, and the thickness of the unfixed toner images on the transfer material is liable to become very large. Because in the site of the process black having such a large image thickness, the pressure applied to the transferred image at fixing by a heating roll becomes large and the toners become liable to be melted as compared with a site having a relatively small image thickness, such as a single-color image, etc., the image gloss becomes high and the site having a high image gloss and the site having a low image gloss are intermixed in the same image, whereby it sometime happens that visual quality feeling is largely lowered. To prevent the occurrence of the phenomenon, an attempt of improving the following property of the heating roll to the transferred image by forming a rubber layer having a low hardness on the surface of the heating roll has been made but lowering of the image quality by the difference of the extents of the image gloss as described above has not yet been completely solved.
Furthermore, because in the portion having such a large image thickness, toners are liable to be melted as described above, there is a problem that an offset of the toners is liable to form to the heating roll; because of the large image thickness, there are problems that the ratio of extending toners to the lateral direction at fixing becomes large, and when an image containing fine lines is fixed, the fine lines become thick to lower the resolution; and because the image thickness after fixing also becomes large, there is a problem that when an external force is applied, the image is liable to be damaged and thus the durability of the image is low.